You see the term often used when describing scopes, but what exactly is parallax? More importantly, what does it mean in terms of rifle scopes?
Parallax describes a situation where the focal plane of the object in the scope is offset from the reticle. If you have parallax, you basically have an optical illusion of sorts that must be corrected.
It’s important to understand that parallax should not be confused with the focus or power of the scope. Parallax compensation changes don’t impact the focus of the reticle or the focus of the image. Parallax causes the planes at which these two objects (the two objects being the reticle and the image) to share the same plane (which is commonly referred to as coindicent).
When looking through a scope, parallax can be seen by shifting your head slightly. If your parallax is not properly adjusted for the target at that specific range, the reticle will change position on the target as your head moves.
Here’s another way to understand parallax: Make a small mark with a pen or pencil on a table or piece of paper on a table. Then put a little bit of water in a clear glass or glass with a clear bottom. Hold the glass just over the mark, and look through the water in the glass at the mark. As you slowly move the glass side to side just a bit, the mark will appear to move as the glass moves. That’s parallax in action, and it happens in scopes as well.
Here is a common example of the adverse effects of parallax which shooters commonly blame on a flawed POI (point of impact) shift or a problem with the scope holding zero. A shooter shooting from a fixed rest puts 3 rounds into a target, and then the next two shots drift right of the target, even though the rifle is strapped in shooting rest. Assuming that the parallax hasn’t been adjusted for the distance to the target, this change in impact may be caused by parallax. Shooters who don’t understand the parallax issue, mistakenly blame the scope.
Over the years, we’ve had plenty of customers who have returned scopes because they “won’t hold zero”. Those scopes were returned to the manufacturer under the warranty and tested at the manufacturer. 99% of the time, those scopes tested fine and did, in fact, hold and maintain zero. I’d bet a week’s pay that parallax was the issue, and the customer just was not aware of how to resolve it.
External parallax adjustments are generally on scopes of more than 10 power, or on scopes that are used at close distances. These external parallax adjustments are called a number of names, but the most common are:
- Adjustable objectives (also commonly called AO) are usually located on the ocular bell of the scope
- Side focus – Usually located on the side of the scope (hence the “side focus” name).
Parallax adjustments are made at the objective lens with a rotating dial marked in incremental distances (typically in yards). Parallax adjustments are specifically tied to distance to the target, and most scopes with parallax adjustments are marked at fixed distances in yards.
The easiest way to solve the parallax issue is to purchase a scope with parallax adjustments, know the range to your target, and adjust the parallax to that range. While that won’t completely eliminate all the parallax, it will reduce it to a minimal level that won’t adversely impact your accuracy.